Typically, an active-matrix liquid crystal display device includes a liquid crystal panel constituted by a liquid crystal layer and two substrates which sandwich the liquid crystal layer. One of these two substrates is formed with a plurality of scanning lines, a plurality of data lines, and a plurality of pixel formation portions arranged in a matrix pattern, with each portion formed correspondingly to one of intersections made by the these scanning lines and the data lines. Each pixel formation portion includes such components as: a thin film transistor (TFT) which has its gate terminal connected to one of the scanning lines that passes a corresponding one of the intersections, and its source terminal connected to one of data lines that passes that intersection; and a pixel capacitance for writing a data signal sent via the data line. The other of the two substrates may be formed with a common electrode which is common to the pixel formation portions. The active-matrix liquid crystal display device further includes: a gate driver (scanning line drive circuit) which drives the scanning lines; and a source driver (data line drive circuit) which drives the data lines.
Although the data signal is sent via the data lines as described above, each data line cannot deliver data signals for a plurality of rows at one time (simultaneously). Therefore, writing of data signals to the pixel capacitances in the pixel formation portions is performed sequentially, row by row, for example. Accordingly, for the purpose of selecting the plurality of scanning lines sequentially for a predetermined period, the gate driver is constituted by a shift register which has a plurality of stages.
Liquid crystal display devices which have such an arrangement as described above may have a problem that even if the user has shut off power supply, the display is not cleared immediately, with an image persisting like a residual image. This happens because, once the power supply to the device is turned off, a discharge path for an electric charge stored in the pixel capacitance is shut off and a residual charge stays in the pixel formation portion. If power supply to the device is turned on while there is a residual charge stored in the pixel formation portion, the residual charge may lower display quality with flickers for example, by causing uneven distribution of impurities. In order to eliminate these problems, conventional solutions include, for example, that all scanning lines are selected at the time when the power is turned off, and a black voltage is applied to all data lines, to allow electric charges on the panel to discharge.
Conventionally, the gate driver is usually mounted at a peripheral region of the liquid crystal panel substrate as an integrated circuit (IC) chip. Increasingly however, the gate driver is formed directly on the substrate, in recent years. Such a gate driver is called “monolithic gate driver”. Also, a panel which includes a monolithic gate driver is called “gate driver monolithic panel”.
In the gate driver monolithic panel, it is impossible to employ the above-described method in regard to releasing the charge from the panel. To this problem, International Laid-Open No. 2011/055584 pamphlet (WO 2011/055584) discloses a liquid crystal display device as follows. In a bistable circuit which constitute a shift register in the gate driver, there is provided a TFT which has: its gate terminal supplied with a clock signal to activate the shift registers; its source terminal connected to a reference potential wiring through which a reference potential is transmitted; and its drain terminal connected to the scanning line. In the arrangement described above, external power supply is shut off, upon which the clock signal is turned to HIGH level to turn ON the TFT, whereas the reference potential is raised from a gate-off potential to a gate-on potential. This increases each scanning line potential to the gate-on potential, whereby the residual charge is discharged in all of the pixel formation portions. Also, International Laid-Open 2010/050262 pamphlet (WO 2010/050262) discloses a technique for eliminating a malfunction caused by a leak from a TFT, as an invention regarding the gate driver monolithic panel.